Hydraulic power generation, wind power generation and photovoltaic power generation which make use of inexhaustible natural energy and help reduce carbon dioxide and improve other environmental problems are getting into the limelight. Out of these, the spread of photovoltaic power generation has been making remarkable progress in recent years as the performance of solar cell modules in power generation efficiency and other respects has been making marked improvements while on the other hand their prices have been declining and the national and local governments has been promoting the business of introducing photovoltaic power generation systems for household use. However, further spread of photovoltaic power generation will require further cost reductions, and to this end research is being continued night and day to work on the improvement of power generation efficiency.
A solar cell module is generally a package formed by protecting a solar cell comprising such solar cell element as silicon, gallium-arsenic and copper-iridium-selenium with a top transparent protective material and a bottom protective substrate material, with the solar cell and the protective materials fixed by use of an encapsulating material. For this reason, any encapsulating material for solar cells is required to have satisfactory transparency so that power generation efficiency will be increased. Encapsulating material for solar cells is also required to have heat resistance so that any troubles such as the flow or deformation of the encapsulating material will not occur even when the temperature rises during the use of the solar cell module. Furthermore, in recent years, as the wall thickness of solar cell element is becoming smaller, encapsulating materials having excellent flexibility are also sought after.
At present, an ethylene-vinyl acetate copolymer having a high vinyl acetate content is used for the encapsulating materials for solar cells in solar cell modules from the viewpoint of flexibility, transparency and other properties. However, because of its inadequate heat resistance, it is necessary to use organic peroxide additionally for such ethylene-vinyl acetate copolymer. For this reason, it is necessary to use a two-step process in which a sheet-like encapsulating material is first prepared from an ethylene-vinyl acetate copolymer containing an organic peroxide and then a solar cell element is sealed with the sheet thus obtained. In the step of making the sheet, it is necessary to mold the sheet at such low temperature that will not cause the decomposition of the organic peroxide and as a result it is impossible to increase the extrusion rate. On top of that, in the step of sealing the solar cell element, it is commonly necessary to carry out a time-consuming two-step bonding process which comprises a step of preliminarily bonding the solar cell over several minutes to score of minutes by use of a laminator and then a step of firmly bonding it over scores of minutes to one hour in an oven at a high temperature at which the organic peroxide is decomposed. Consequently, it is troublesome and requires much time to produce a solar cell module, which in turn constitutes a factor in increasing the manufacturing cost.    Reference 1: Japanese Patent Publication SHO 2-40709